Evaporative fuel vapors generated in a vehicle fuel system are adsorbed by activated carbon (charcoal) in a canister and later purged and consumed during engine combustion. During a typical period when the vehicle is parked with the engine off, called a soak, the canister becomes only partially loaded with hydrocarbon vapor, or partially saturated with hydrocarbon vapor from the fuel tank. The partially saturated canister may experience several hours of soak before it is purged or reloaded with vapors. Partially loaded or purged canisters left overnight have shown far less available vapor adsorption capacity and a higher tendency to have hydrocarbon vapor filter out of the canister through the atmospheric vent to yield breakthrough emissions, than similar canisters tested immediately.